1. Field of the Invention
The present invention relates to hard films excellent in wear resistance and heat resistance to be applied to wear-resistant objects, such as cutting tools, sliding members, dies and molds. More specifically, the present invention relates to useful hard films excellent in wear resistance and heat resistance capable of being formed at low temperatures on base objects, such as cutting tools, sliding member and the like, without spoiling the characteristics of the base objects.
2. Description of the Related Art
Although the hard films of the present invention are versatile and are applicable to diverse uses as mentioned above, the hard films of the present invention will be described as applied mostly to cutting tools by way of example.
Cutting tools and sliding members required to have excellent wear resistance and sliding characteristics are formed by coating the surfaces of base objects of high-speed tool steels and cemented carbides with hard films of titanium nitride and titanium-aluminum nitrides by a physical vapor deposition process (hereinafter referred to as xe2x80x9cPVD processxe2x80x9d) and chemical vapor deposition process (hereinafter referred to as xe2x80x9cCVD processxe2x80x9d). Sometimes, the cutting edges of cutting tools are heated at high temperatures exceeding 1000xc2x0 C. Therefore, hard films of alumina are formed on such cutting tools to ensure the heat resistance of the cutting tools.
Although the crystal structure of alumina is dependent on deposition temperature, alumina except for alumina of corundum crystal structure is metastable. However, the crystal structure of a metastable alumina film formed on the edge of a cutting tool, whose temperature varies in a wide temperature range between an ordinary temperature and 100xc2x0 C., changes, cracks develop in the alumina film and, sometimes, the alumina film comes off the cutting tool. Only an alumina film of corundum crystal structure once formed by a CVD process that heats a base object at a temperature not lower than 1000xc2x0 C. maintains thermodynamically stable structure regardless of temperature. Therefore, it is very effective in providing cutting tools with heat resistance to coat the cutting tools with an alumina film of corundum crystal structure.
The alumina film of corundum crystal structure cannot be formed unless the base object is heated at a high temperature not lower than 1000xc2x0 C. Therefore such an alumina film can be formed on limited base objects because some base objects soften and loose an aptitude for base objects for forming wear-resistant objects when exposed to high temperatures not lower than 1000xc2x0 C.
A very hard (Al, Cr)2O3 mixed crystal film formed at temperatures not higher than 500xc2x0 C. is mentioned in JP5-208326A. However, Cr lying in the surface of the mixed crystal film is liable to react with Fe of a workpiece of a metal containing Fe as a principal component during machining and, consequently, the mixed crystal film wears out rapidly to shorten its life.
A method of forming films of alumina of corundum crystal structure at temperatures not lower than 750xc2x0 C. by reactive sputtering using a high-power pulse power source of 11 to 17 kW is mentioned in O. Zywitzki, G. Hoetzsch, et al., Surf. Coat. Technol., 86-87 (1996) 640-647. This method of forming films of alumina of corundum crystal structure inevitably needs a large pulse power source and needs to heat a base object at temperatures not lower than 750xc2x0 C. Consequently, high-speed tool steels generally used for forming base objects of cutting tools are softened and deteriorate the characteristics of the base objects.
Cutting tools prevalently used at present are formed by coating the surfaces of base objects with a wear-resistant film of titanium nitride, titanium carbide or carbonitride, and forms a film of alumina of corundum crystal structure over the wear-resistant film. Recently, films of titanium nitride, titanium carbide and carbonitride have been gradually replaced by films of titanium-aluminum mixed nitride (hereinafter referred to as xe2x80x9cTiAlNxe2x80x9d) having excellent wear-resistant property, and TiAlN films have been applied to cutting tools and such.
However, whereas the TiAlN film can be formed only by an arc ion plating process (hereinafter referred to as xe2x80x9cAIP processxe2x80x9d), i.e., one category of PVD processes, the alumina film of corundum crystal structure can be formed only by a CVD process. Therefore, to obtain a laminated film of an alumina film and a TiAlN film, the alumina film and the TiAlN film need to be formed successively by using a CVD system and a PVD system, respectively. Consequently, the production efficiency of the processes is very low. Thus, it has been desired to establish techniques capable of efficiently, continuously forming an alumina film of corundum crystal structure, a TiAlN film and other useful films.
The present invention has been made in view of the foregoing circumstances and it is therefore an object of the present invention to provide a hard film having excellent heat resistance and wear resistance and capable of efficiently formed on a base object at a low temperature that will not deteriorate the characteristics of the base object.
According to a first aspect of the present invention, a hard film comprising an oxide film of corundum crystal structure having a lattice constant in the range of 4.779 to 5.000 xc3x85 and a thickness of 0.005 xcexcm or above; and an alumina film of corundum crystal structure formed on one surface of the oxide film of corundum crystal structure.
Desirably, the oxide film is a film of Cr2O3, (Fe, Cr)2O3 or (Al, Cr)2O3, (Fe, Cr)2O3 is (Fex, Cr(1xe2x88x92x))2O3, where 0xe2x89xa6xxe2x89xa60.54, (Al, Cr)2O3 is (Aly, Cr(1xe2x88x92y))2O3, where 0xe2x89xa6yxe2x89xa60.90.
Preferably, a film of a mixed nitride of one or some of Ti, Cr and V, and Al is formed directly on the other surface of the oxide film of corundum crystal structure or on an intermediate layer formed on the other surface of the oxide film of corundum crystal structure, and the intermediate layer is a film of (Alz, Cr(1xe2x88x92z))N, where 0xe2x89xa6zxe2x89xa60.90.
The present invention includes a wear-resistant object formed by coating a base object with any one of the foregoing hard films with the alumina film of corundum crystal structure facing out.
A method of forming a wear-resistant object, in a second aspect of the present invention comprises the steps of forming an alumina film of corundum crystal structure, forming the oxide film, forming the mixed nitride film and, if necessary, forming an intermediate layer by PVD processes. Preferably, the film of (Aly, Cr(1xe2x88x92y))2O3, where 0xe2x89xa6yxe2x89xa60.90 is formed by forming the intermediate layer of (Alz, Cr(1xe2x88x92z))N, where 0xe2x89xa6zxe2x89xa60.90, and converting a surface part of the intermediate layer into an oxide layer by an oxidation process that holds the base object at a temperature not lower than 450xc2x0 C. in an oxygen atmosphere. Preferably, the base object is heated at temperatures not lower than 300xc2x0 C. to form the aluminum film of corundum crystal structure.
The oxide film of corundum crystal structure having the lattice constant specified by the present invention formed before the formation of the alumina film of corundum crystal structure enables the formation of the alumina film of corundum crystal structure excellent in heat resistance and wear resistance. The hard film forming method of forming the foregoing hard film is capable of forming the alumina film of corundum crystal structure on diverse base objects to provide the base object with excellent heat resistance and wear resistance.